The present invention relates to a lithium ion secondary cell, and more particularly, to a lithium ion secondary cell having a large capacity and high energy density and capable of application in fields where there is high demand for maintenance-free properties of the products or system, such as in the manufacture of electric automobiles, load leveling of electric power, etc.
To meet the need for miniaturization and light-weight of electronic devices in recent years, there have been developed as a power source, lightweight, miniaturized lithium ion secondary cells and these have been practically used in certain electronic devices such as handheld video cameras, portable personal computers and the like. However, the maximum capacity of these lithium ion secondary cells in practical use is around 5-20 Wh and these are mostly cylindrical in shape.
On the other hand, because of the increasing demand for protecting the environment and saving resources, there has been increasing interest shown in electric automobiles, and also in the need to use load leveling of electric power for effective utilization of nighttime power supply. These demands require the development of large-capacity and low-cost maintenance-free secondary cells.
The lead accumulators widely used in these fields at present are low in output of energy and also inconvenient for use because of heavy weight. Also, these lead accumulators involve the problems relating to maintenance such as the necessity of water replenishment, and their charge-discharge cycle life is as short as about 600 cycles, resulting in a high cost for these batteries. Nickel-cadmium batteries are also used, but their use is limited as the nickel-cadmium batteries are low in energy density, and high in manufacturing cost compared with lead accumulators.
Other types of batteries such as nickel-zinc batteries and sodium-sulfur batteries have been experimentally used for electric automobiles, but they involve their own problems such as short charge-discharge cycle life (nickel-zinc battery) and high possibility of causing dangerous conditions (sodium-sulfur battery).
A Lithium ion secondary cell is suited for these applications because of its high energy density and its structural features of being a closed type and maintenance-free, but none with a large capacity has ever been commercially produced. For application to these uses for electric automobiles, the battery is required to have a capacity on the order of 1,000-5,000 Wh, which is more than 100 times the capacity now available.
The lithium ion secondary cells in practical use are mostly of the cylindrical type, but those with a large capacity of the order of 1,000 to 5,000 Wh required for electric automobiles, load leveling, etc., must be square-type cells comprising an assembled cell of two or more of series-connected 3-4 V single cells of a structure in which positive electrodes comprising metal foils coated with an active substance and negative electrodes comprising metal foils coated with an active substance are stacked alternately by means of a separator interposed therebetween. Such square type lithium ion secondary cells have not yet been put to practical use, nor are there such cells commercially available with a large size, and high toughness, vibration resistance and impact resistance sufficient for application to electric automobiles.
For use of lithium ion secondary cells as the large-capacity batteries applicable to electric automobiles or load leveling of electric power, it is necessary first of all to enlarge their electric capacity.
The lithium ion second batteries used for such purposes are usually designed to be a square-type cell comprising an assembled cell of series-connected single cells each of which consists of several ten to about 100 electrodes associated in laminate so that the positive electrode and the negative electrode are arranged alternately by means of the separator interposed therebetween.
Especially, in application to electric automobiles, the said batteries are required to be enlarged in size and to have high toughness, vibration resistance and impact resistance. This requires the electrodes themselves to have long charge-discharge cycle life as well as high toughness, vibration resistance and impact resistance. Also, the structure of the single cells comprising a stacked assembly of such electrodes needs to be designed to have equally high toughness, vibration resistance and impact resistance.
It is demanded to enlarge the size of the lithium ion secondary cells and to enhance toughness as well as vibration resistance and impact resistance of the lithium ion secondary cells.
In case of the enlargement of a lithium ion secondary cell, such lithium ion secondary cell involves a problem that the heat generated in the inside of the cell during charge and discharge tends to accumulate in the battery, thereby causing a rise of internal temperature of the cell, even above the permissible level. So, there needs to be a release of such heat to the outside of the cell. A need for a secondary cell which can endure rapid charge and discharge is especially high in application to electric automobiles. The conventional lithium ion secondary cells have the problem that heat is accumulated in the inside of the cell as rapid cycles of charge and discharge are repeated, causing a rise of internal temperature of the cell above the allowable level.
As a result of the present inventors' earnest studies for solving the above problems, it has been found that:
(I) a lithium ion secondary cell comprising a single cell comprising positive electrodes composed of a metallic material (metal sheet) coated with a positive electrode active substance and negative electrodes composed of a metallic material (metal sheet) coated with a negative electrode active substance are stacked alternately by means of separators interposed therebetween, the lugs of the metallic materials of the respective electrodes being electrically connected to the respective conductors separately, thereby forming a collector, (i) wherein the lugs of the metallic materials of the positive and negative electrodes are respectively bunched together and clamped with the conductors, the edges of the said lugs of the electrodes and the said conductors are welded to form the collector so that electric current can be taken out through the said conductors, or (ii) wherein conductive spacers for keeping the distance between the electrodes constant are disposed between the lugs of the metallic materials of the respective electrodes in the interval of the positive or negative electrode substance and the welded portion and secured in position for electrically connecting the electrodes of each single cell separately to form the collector so that electric current can be taken out through the said spacers; or PA1 (II) a lithium ion secondary cell comprising an assembled cell composed of two or more of series-connected single cells in each of which positive electrodes composed of a metallic material (metal sheet) coated with a positive electrode active substance and negative electrodes composed of a metallic material coated with a negative electrode active substance are stacked alternatively by mean of separators interposed therebetween, in which the lugs of the metallic materials of the respective electrodes are electrically connected to the conductors separately, thereby forming a collector, and conductors electrically and thermally connected to the collector are so arranged that at least one conductor per cell for each electrode is extended, directly from each single cell, to the outside of the cell through the wall of its case so that electric current can be taken out through the said conductors while allowing release of the heat accumulated in the cell during charge and discharge to thereby prevent rise of internal temperature of the cell, PA1 the lithium ion secondary cells (I) and (II) have high toughness, vibration resistance and impact resistance, and are capable of effectively preventing rise of internal temperature of the cell. On the basis of the finding, the present invention has been attained. PA1 positive electrodes composed of a metallic material (metal sheet) coated with a positive electrode active substance; negative electrodes composed of a metallic material (metal sheet) coated with a negative electrode active substance; separators interposed between said positive and negative electrodes; lugs of said metallic materials where said active substance is not coated; conductors electrically connected to said lugs of the positive and negative electrodes separately; and conductive spacers disposed between said lugs of the positive and negative electrodes separately for maintaining constant the spacing between the positive and negative electrodes, PA1 said positive and negative electrodes being assembled in laminate alternately, and the spacers disposed between the lugs of the positive electrodes and the spacers disposed between the lugs of the negative electrodes being respectively secured in position so as to electrically connect the electrodes of the single cells so that electric current may be taken out through said spacers. PA1 positive electrodes composed of a metallic material (metal sheet) coated with a positive electrode active substance; negative electrodes composed of a metallic material (metal sheet) coated with a negative electrode active substance; separators disposed between said positive and negative electrodes; lugs of said metallic materials where said active material is not coated; and conductors connected separately to said lugs of the positive and negative electrodes, PA1 the conductors electrically and thermally connected to said collectors being so arranged that at least one conductor per cell for each electrode is extended directly from each cell to the outside of the cell container through the wall thereof so that electric current can be taken out through said conductors while allowing release therethrough of the heat accumulated in the cell during charge and discharge, thereby preventing a rise in temperature in the cell.